Friday, 11 March 2016

750 GeV: the bigger picture

This Thursday the ATLAS and CMS experiments will present updated analyses of the 750 GeV diphoton excess. CMS will extend their data set by the diphoton events collected in the periods when the detector was running without the magnetic field (which is not essential for this particular study), so the amount of available data will slightly increase. We will then enter the Phase-II of the excitement protocol, hopefully followed this summer by another 4-th-of-July-style discovery. To close the Phase-I, here's a long-promised post about the bigger picture. There's at least 750 distinct models that can accommodate the diphoton signal observed by ATLAS and CMS. However, a larger framework for physics beyond the Standard Model it which these phenomenological models can be embedded is a more tricky question. Here is a bunch of speculations.

Whenever a new fluctuation is spotted at the LHC one cannot avoid mentioning supersymmetry. However, the 750 GeV resonance cannot be naturally interpreted in this framework, not the least because it cannot be identified as a superpartner of any known particles. The problem is that explaining the observed signal strength requires introducing new particles with large couplings, and the complete theory typically enters into a strong coupling regime at the energy scale of a few TeV. This is not the usual SUSY paradigm, with weakly coupled physics at the TeV scale followed by a desert up to the grand unification scale. Thus, even if the final answer may still turn out to be supersymmetric, it will not be the kind of SUSY we've been expecting all along. Weakly coupled supersymmetric explanations are still possible in somewhat more complicated scenarios with new very light sub-GeV particles and cascade decays, see e.g. this NMSSM model.

Each time you see a diphoton peak you want to cry Higgs, since this is how the 125 GeV Higgs boson was first spotted. Many theories predict an extended Higgs sector with multiple heavy scalar particles, but again such a framework is not the most natural one for interpreting the 750 GeV resonance. There are two main reasons. One is that different Higgs scalars typically mix, but the mixing angle in this case is severely constrained by Higgs precision studies and non-observation of 750 GeV diboson resonances in other channel. The other is that, for a 750 GeV Higgs scalar, the branching fraction into the diphoton final state is typically tiny (e.g., ~10^-7 for a Standard-Model-Higgs-like scalar) and a complicated model gymnastics is needed to enhance it. The possibility that the 750 GeV resonance is a heavy Higgs boson is by no means excluded, but I would be surprised if this were the case.

It is more tempting to interpret the diphoton resonance as a bound state of new strong interactions with a confinement scale in the TeV range. We know that the Quantum Chromodynamics (QCD) theory, which describes the strong interactions of the Standard Model quarks, gives rise to many scalar mesons and higher-spin resonances at low energies. Such a behavior is characteristic for a large class of similar theories. Furthermore, if the new strong sector contains mediator particles that carry color and electromagnetic charges, the production in gluon fusion and decay into photons is possible for the composite states, see e.g. here. The problem is that, much as for QCD, one would expect not one but an entire battalion of resonances. One needs to understand how the remaining resonances predicted by typical strongly interacting models could have avoided detection so far.

One way this could happen is if the 750 GeV resonance is a scalar that, for symmetry reasons, is much lighter than most of the particles in the strong sector. Here again our QCD may offer us a clue, as it contains pseudo-scalar particles, the so-called pions, which are a factor of 10 lighter than the typical mass scale of other resonances. In QCD, pions are Goldstone bosons of the chiral symmetry spontaneously broken by the vacuum quark condensate. In other words, the smallness of the pion mass is protected by a symmetry, and general theorems worked out in the 60s ensure the quantum stability of such an arrangement. The similar mechanism can be easily implemented in other strongly interacting theories, and it is possible to realize the 750 GeV resonance as a new kind of pion, see e.g. here. Even the mechanism for decaying into photons -- via chiral anomalies -- can be borrowed directly from QCD. However, the symmetry protecting the 750 GeV scalar could also be completely different that the ones we have seen so far. One example is the dilaton, that is a Goldstone boson of a spontaneously broken conformal symmetry, see e.g. here. This is a theoretically interesting possibility, since approximate conformal symmetry often arises as a feature of strongly interacting theories. All in all, the 750 GeV particle may well be a pion or dilaton harbinger of new strong interactions at a TeV scale. One can then further speculate that the Higgs boson also originates from that sector, but that is a separate story that may or may not be true.

Another larger framework worth mentioning here is that of extra dimensions. In the modern view, theories with the new 4th dimension of space are merely an effective description of strongly interacting sectors discussed above. For example, the famous Randall-Sundrum model, with the Standard Model living in a section of a 5D AdS5 space, is a weakly coupled dual description of strongly coupled theories with a conformal symmetry and a large N gauge symmetry. These models thus offer a calculable way to embed the 750 GeV resonance in a strongly interacting theory. For example, the dilaton can be effectively described in the Randall-Sundrum model as the radion - a scalar particle corresponding to fluctuations of the size of the 5th dimension, see e.g. here. Moreover, the Randall-Sundrum framework provides a simple way to realize the 750 GeV particle as a spin-2 resonance. Indeed, the model always contains massive Kaluza-Klein excitations of the graviton, whose couplings to matter can be much stronger than that of the massless graviton. This possibility have been relatively less explored so far, see e.g. here, but that may change next week...

Clearly, it is impossible to say anything conclusive at this point. More data in multiple decay channels is absolutely necessary for a more concrete picture to emerge. For me personally, a confirmation of the 750 GeV excess would be a strong hint for new strong interactions at a few TeV scale. And if this is indeed the case, one may seriously think that our 40-years-long brooding about the hierarchy problem has not been completely misguided...

38 comments:

Sven
said...

From the theory side I think it is very important that the authors of these O(750) models not only ensure that they can produce the signal (which most did), but also that their solution is in agreement with all other experimental constraints (which not everybody did). Furthermore it would be "helpful" if the authos analyzed how their solution can be distinguished from others and how it can be tested in the future. I think there is quite some homework to be done.

As for the dilaton case (as well as for its 5D dual picture in RS), I'd like just to point out that it's not a totally separate assumption whether the Higgs originates from the same strong/composite/conformal dynamics or not. In fact, I think that the Higgs in that scenario cannot be (fully) composite, as otherwise the dilaton BR into the Higgs (read the longitudinal Ws) would largely dominate the other channels. A super large (~100 or more n proper units) trace anomaly would naively change this conclusion but in fact it's of no help as it would make the dilaton super heavy, too. All in all, the composite PGB scenario looks more motivated than the dilaton to me at this stage,, as the former can serve another purpose at the same time, namely to solve the hierarchy problem in the Higgs sector of the SM.

So much depends on the details of what is found. For instance if this thing resolves into a double resonance side by side, the likely favored theories significantly changes. For that reason, it's hard to do a big picture here with any confidence.

As Moriond starts tomorrow, I would expect some proper timetable somewhere. Did they really forget that?

There is also a talk "Probing the Higgs force in atoms" on Tuesday -> http://resonaances.blogspot.de/2016/01/higgs-force-awakens.html

Concerning the 750 GeV peak: okay, a few more events from CMS (magnet off means no particle flow algorithm, which probably degrades calorimeter resolution a bit due to pileup), and probably many additional cross-checks, but I would not expect something really surprising.

You cannot expect specific surprising things, but you can expect that something surprising comes up.

As an example, I expect that LIGO/VIRGO will find something surprising. And to take that one step further: I would be surprised if they do not find anything surprising. I assign a low probability to all results I can imagine today.

Anon, the point is that a scalar is not enough. You need more degrees of freedom to produce and decay it in agreement with experimental data, and these new particles need to have fairly large couplings. From that point on it is my subjective feeling that strong interactions near the TeV scale provide the neatest UV completion.

That set-up is perfect as an effective theory at the energy scale of order 750 GeV. However, it's not a UV complete model, and at the scale of at most ~10 TeV it will cease to be a healthy quantum theory (the phi* F^2 coupling makes scattering amplitudes grow with energy and they eventually lose perturbative unitarity). All existing UV completions I'm aware of involve new particles with very large couplings that typically run to a Landau pole not far above the TeV scale.

I think that the requirement that the theory be UV complete is overshooting the problem. We already know that the SM is just an effective theory (of some nonrenormalizable fundamental non-QFT theory at the Planck scale), so why is it a problem to have phi*F^2 term in the effective low-energy (TeV scale) model? QED also has a Landau pole, but everybody expects QED to become invalid way before the pole.

Why do you require any effective theory to be perturbatively unitary to begin with? I see that requirement only as a leftover principle from the days when people thought that QFT can be a fundamental theory of everything. Am I missing something?

As an effective framework, the SM eventually runs into same UV problems: QED is plagued by the Landau pole, perturbative unitarity is lost in high-energy Higgs scattering on polarized W bosons...and so on.

I do not think that we currently have a robust clue on how to build a UV complete theory, aside from a myriad of speculative scenarios. To give one example, there are indications that Technicolor is dead by now, yet some feel that it may be revived if new particles with strong couplings start to show up soon.

vmarko, wouldn't such an effective theory have quite limited predictive capacity? This new resonance if confirmed would be a clear indication of some BSM physics - but how much insight into it we would gain if the model is limited to replication of already observed results in a limited energy range?

I agree that the predictive power of the model is limited. But that's an aesthetic criterion. Certainly some other ideas could work better or have more predictions, but that's a matter of taste, at least until new data arrives.

What I was trying to say is that I don't see any of the theoretical arguments put forward against the model by Jester to be showstoppers. Lack of UV-completeness and perturbative unitarity, the presence of the Landau pole, etc., are not good enough reasons to completely dismiss the model, IMO, regardless of any predictive power (or lack thereof).

vmarko, I've no intent to go too philosophical but looks like the distinction is between an effective model vs. new theory (and it gets somewhat confused by using "model" in both cases). Clearly there can be a place for an effective model but in my view it can't be seen as a new theory until it comes up with some new insight on the nature of the observed event (if and hopefully ;)

Hello Jester! I can not find the talk on wednesday, 16.03.2016 at the Moriond Meeting. About which one are you talking about - are you sure you dont mean the thursday session on: 17:00 - 18:30 Beyond SM, Diphoton searches in ATLAS/CMS and Interpreting the 750 GeV digamma excess: a review?Thanks!

Hey Jester, how come we don't hear from you about the rumors pre-Moriond talks (on Thursday)? One of them says ATLAS will present diphoton reanalyses that push up the significance by 0.6. Another that ATLAS reaches 5sigma(local).

ATLAS cancelled, CMS not sharing the stuff we care about... so we won't hear anything this week after all? Jester, any chance you can seduce a researcher at each experiment and plant a listening device on them?

Hmm, about the comment of RBS... a new force does not need a new set of quarks, does it? It could be done with the six we have. Even better, if it can be done with only five quarks (explaining-excluding the peculiarity of the top quark), the number of SU(5)flavour pairs provides three generations of "fake squarks" and "fake sleptons" which phenomenologists could enjoy.

And in any case, if the new strong force is not a chiral force, we do not need extra leptons to cancel anomalies, do we?

Xezlec, as far as I know, CMS will show their new 750 GeV diphoton analysis today (slight increase of significance as compared to the December analysis). ATLAS should also come out of the closet at some point, once the analysis is properly vetted.

Hey Jester! Could you please summarize the ATLAS+CMS presentations from today? In a tweet you write that CMS data increase significancy, and ATLAS didnt present new data; but I have seen the slides by ATLAS. I think i am a bit confused. Thanks :)

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Résonaances is a particle physics blog from Paris. It's about the latest news and gossips in particle physics and astrophysics. The posts are often spiced with sarcasm, irony, and a sick sense of humor. The goal is to make you laugh; if it makes you think too, that's entirely on your own responsibility...